Thermal Gradient Driven Variable Optical Attenuator With On-Chip CNT Absorber

A thermal gradient driven variable optical attenuator (VOA) has been designed, fabricated and characterized. This extremely simple device consists only one straight waveguide and one heater electrode. The narrow bar waveguide design causes the light field to expand in the cladding and become susceptible to temperature gradient generated by the top heater. To solve the problem with stray light, a slot filled with carbon nanotubes (CNTs) is integrated on the chip parallel to the waveguide. The selected mixture of CNTs can provide over 99.9% absorption of light from 1500 nm to 1630 nm regardless of polarization. The final device has an insertion loss lower than 0.88 dB including fiber coupling loss at both facets, and an attenuation level larger than 37 dB at 52.5 mW heater power over the entire wavelength window for both polarizations. This work demonstrates a practical VOA device to be readily connected with other polymer waveguide components, but also proves the feasibility to apply CNTs as effective stray light absorber in photonics integrated circuits.